Solid imaging compositions for preparing polyethylene-like articles

ABSTRACT

This invention discloses compositions adapted to produce, through solid imaging means, excellent quality objects having material properties that simulate the look and feel of polyethylene articles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/113,271, filed Jul. 10, 1998 now U.S. Pat. No. 6,287,748, thecontents of which are hereby incorporated in their entirety byreference. U.S. application Ser. No. 09/113,271 has issued on Sep. 11,2001, as U.S. Pat. No. 6,287,748.

FIELD OF THE INVENTION

This invention discloses compositions adapted to produce, through solidimaging means, excellent quality objects having material properties thatsimulate the look and feel of polyethylene articles.

BACKGROUND OF INVENTION

In the field of liquid-based solid imaging, alternatively known asstereolithography, compositions have been developed which are capable ofgenerating solid objects having the properties of epoxies and/oracrylates. Solid imaging generated objects made from previous epoxyand/or acrylate compositions provide a prototypical representation ofthe physical shape of plastic articles made on a production basis out ofmaterials such as ABS, nylon, polyethylene, polypropylene, etc. However,such compositions lack the material properties that give users of theprototypes a sense of look and feel for the object when produced in theproduction material. Such a lack of look and feel accuracy in productprototyping is not just an aesthetic issue. The look and feel of aprototype also has significant engineering, design, packaging, labeling,and advertising implications.

For example, squeeze bottles, such as those used for dispensing dishsoap, are designed to be attractive in shape, easy to grasp, and easy tosqueeze. Typically such bottles are made from polyethylene typepolymers. Previous epoxy and/or acrylate compositions used in solidimaging were capable of producing articles that have the same attractiveshape. However, the stiffness of the prototyped articles made from thesematerials was likely to mislead the designer and evaluator of thearticle relative to such issues as, for example, wall thickness andsurface radius design. For example, a commercial solid imaging resin,Somos® 2100 (E. I. DuPont De Nemours, Inc., Wilmington, Del.), producesarticles having lower stiffness than polyethylene. Bottles made fromthis material do not provide adequate resistance to squeezing such thatenough friction occurs between a person's fingers and the bottle. Aperson holding a Somos® 2100 prototype bottle of liquid soap is likelyto squeeze too hard in order to generate enough friction to keep thebottle from slipping. As a consequence, the soap is likely to bedispensed prematurely. The designer of the bottle might then be misledinto making the wall thickness of the bottle greater in order to improveits stiffness. But such a design change would lead to a bottle that istoo stiff when manufactured with polyethylene. Similar problems aregenerated when other much stiffer epoxy and/or acrylate compositions areused to prototype articles such as bottles. A designer might be led todecrease the wall thickness of the bottle due to the stiffness. Or forexample, since sharper radii may not feel as comfortable duringsqueezing when stiffer materials are used, the designer may be misledinto re-designing the bottle with greater radii. This may affect thebottle squeezability when manufactured in polyethylene or may reduce theaesthetic appeal of the bottle shape.

Other examples may be made regarding the importance of appearance of anarticle when made out of certain materials. For example, use of atransparent prototype composition or an overly opaque composition maymislead those viewing the article into incorrect assumptions regardingappropriate packaging, labeling, coloring, and advertising of a product.

Other considerations when trying to utilize solid imaging forprototyping include photospeed, resistance to humidity, low potentialfor hydrolysis, similar coefficient of friction, dimensional accuracy,ability to span without supports during fabrication, and wide processlatitude.

Japanese Patent Application Hei 2-75618 describes epoxy and acrylatecompositions for use in optical molding. The compositions contain atleast 40 wt % of alicyclic epoxy resin with at least two epoxy groups ineach molecule.

U.S. Pat. No. 5,476,784 describes cationic epoxy and acrylatecompositions for use in solid imaging. The compositions may comprisefrom 5-40% by weight of at least one OH-terminated polyether, polyesteror polyurethane. In the examples given, the polyether polyolsformulations provide lower elongation at break properties that theelongation at yield properties of most low-density polyethylenes.Additionally, the patent teaches that the epoxy content is to be from 40to 80% of the formulation by weight. Compositions made with this epoxycontent are likely to produce cured articles having a higher modulusthan that of polyethylene.

SUMMARY OF INVENTION

This invention discloses photosensitive compositions comprising;

(a) about 20-40% by weight of an epoxide-containing material;

(b) about 5-40% by weight of acrylic material selected from aromaticacrylic material, cycloaliphatic acrylic material, or combinationsthereof;

(c) about 5-50% by weight of a reactive hydroxyl-containing material;

(d) at least one cationic photoinitiator; and

(e) at least one free-radical photoinitiator;

with the proviso that upon exposure to actinic radiation an article isproduced having the following properties:

(i) a tensile break before yield stress or a tensile yield stressgreater than 13 N/mm2;

(ii) a tensile modulus in the range of about 180 to about 850 N/mm2;

(iii) a tensile break elongation before yield or a tensile yieldelongation greater than 6%; and

(iv) a notched Izod impact strength greater than 50 J/m.

The invention also relates to a process for forming a three-dimensionalarticle, said process comprising the steps:

(1) coating a thin layer of a composition onto a surface;

(2) exposing said thin layer imagewise to actinic radiation to form animaged cross-section, wherein the radiation is of sufficient intensityto cause substantial curing of the thin layer in the exposed areas;

(3) coating a thin layer of the composition onto the previously exposedimaged cross-section;

(4) exposing said thin layer from step (3) imagewise to actinicradiation to form an additional imaged cross-section, wherein theradiation is of sufficient intensity to cause substantial curing of thethin layer in the exposed areas and to cause adhesion to the previouslyexposed imaged cross-section;

(5) repeating steps (3) and (4) a sufficient number of times in order tobuild up the three-dimensional article;

with the proviso the three-dimensional article has the followingproperties:

(i) a tensile break before yield stress or a tensile yield stressgreater than 13 N/mm2;

(ii) a tensile modulus in the range of about 180 to about 850 N/mm2;

(iii) a tensile break elongation before yield or a tensile yieldelongation greater than 6%; and

(iv) a notched Izod impact strength greater than 50 J/m.

The invention also relates to the above process wherein the compositioncomprises:

(a) about 20-40% by weight of an epoxide-containing material;

(b) about 5-40% by weight of an aromatic or cycloaliphatic acrylicmaterial;

(c) about 5-50% by weight of a reactive hydroxyl-containing material;

(d) at least one cationic photoinitiator; and

(e) at least one free-radical photoinitiator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Liquid based Solid Imaging is a process wherein a photoformable liquidis coated into a thin layer upon a surface and exposed imagewise toactinic radiation such that the liquid solidifies imagewise.Subsequently, new thing layers of photoformable liquids are coated ontoprevious layers of liquid or previously solidified sections. Then thenew layer is exposed imagewise in order to solidify portions imagewiseand in order to induce adhesion between portions of the new hardenedregion and portions of the previously hardened region. Each imagewiseexposure is of a shape that relates to a pertinent cross-section of aphotohardened object such that when all the layers have been coated andall the exposures have been completed, an integral photohardened objectcan be removed from the surrounding liquid composition.

One of the most important advantages of the solid imaging process is theability to rapidly produce actual objects that have been designed bycomputer aided design. A significant amount of progress has been madewith compositions and processes that have been adapted to improve theaccuracy of the objects produced. Also, composition developers have madesignificant progress toward improving individual properties such as themodulus or deflection temperature of the photohardened objects. However,attempts to simulate a particular set of physical properties of a commonmanufacturing material to such a degree that the simulation materialcould be easily mistaken for the simulated material, based upon look andfeel properties, have been unsuccessful.

During the development of the compositions disclosed herein, it wasnoted that substantial changes in the look and feel of articlesfabricated by the liquid solid imaging process could be attained byslight alterations in component concentration. Surprisingly it was foundthat by making these alterations in composition, articles could be madethat had the look and feel of articles manufactured from polyethylenematerials. Within the field of liquid solid imaging such a discovery isa first in that previous commercial compositions did not make articlesthat elicited a similar look and feel sense with regard to any othercommon plastic. It was then recognized that by tailoring thecomposition, the properties of polyethylene manufactured articles couldbe simulated. This potential therefore solved an oft expressed butunfilled need to produce prototypes that not only had the appearance ofdesired objects but also material properties that simulated the look andfeel of the materials out of which production objects were destined tobe manufactured.

In order to simulate a material in terms of look and feel, it isnecessary to decide upon the appropriate appearance factors and physicalproperties. For example, in the field of liquid solid imaging the mostcommonly quoted fully cured part physical properties are the tensilestress, tensile modulus, elongation at break, flexural stress, flexuralmodulus, impact strength, hardness, and deflection temperature. Some ofthese physical properties, such as for example elongation at break, arenot something that can be felts unless the material is deformed. Suchphysical properties are therefore not indicative of a good simulationmaterial property.

In some cases, the characteristics of a material that serve to definethe look and feel properties of a particular material are difficult todefine. This is especially so in the case of how a materials looks.However, in the case of the instant invention a deliberate compositionalchoice was made such that articles fabricated through solid imagingmeans, when given various amounts of exposure to actinic radiation, hada similar color and light scattering characteristic as various grades ofpolyethylene. It was also found that changing the actinic exposure canalso modify the feel properties of the articles manufactured from thecomposition by the solid imaging process. That is, when lower actinicexposures are given during the solid imaging process, the articles feelmore like lower density polyethylene articles but when higher actinicexposures are given, the articles feel more like medium densitypolyethylene articles.

A stress-strain curve, also known as a deformation curve, shows therelationship between the stress or load on a sample and the strain ordeformation that results. The tensile stress-strain curve results whenthe sample is under tension. The flexural stress-strain curve is aresult of bending the sample. It is considered here that the tensileproperties are best representative of how the articles feel.

At the beginning of stress, the stress-strain relationship is roughlylinear. The “tensile modulus” (or the flexural modulus) is defined asthe slope of the curve in this linear region. As the sample continues tobe stressed, the stress-strain curve tends to become less linear until amaximum is reached. This maximum point is called the “yield point” andis generally defined as the region where a large increment of extensionoccurs under constant load. The “yield stress” is the stress at theyield point. The “elongation at yield” is the percent elongation at theyield point. Some materials exhibit increased stress and straincapability beyond the yield point. The point at which the sample breaksis where the values of “break stress” and “%elongation at break” can beascertained. Some samples may break prior to the yield point or at theyield point. In the case of break before yield, only the break stressand % elongation at break values are reported. It is possible for thebreak stress to be a lower or higher value than the yield stress. Alltensile properties as discussed herein were measured according to ASTMTest D-638M, except that for the example formulations and thecomparative formulations the humidity was not controlled.

By far, the most important property that relates to what is felt whenhandling a material, is the tensile modulus. This is representative ofthe feeling of stiffness.

A second important property is that of the elongation of the material.When a simulation of a material is handled and flexed, it should notbreak or permanently distort if the material being simulated does notbreak or distort with such handling. With plastics there is considerabledebate relating to the point at which a sample under stress transitionsfrom an elastic mode to a plastic mode of behavior. However, most wouldagree that when a material begins to yield, its behavior is plastic andthat any handling that brings a sample past its yield point will leavethe sample permanently distorted. For the purposes of this invention thetensile elongation at yield serves to help define this aspect of thefeel of a material. If a material breaks before its yield, it must havea tensile elongation at break that is greater than or equal to thetensile elongation at yield of the material being simulated or it is notan acceptable simulation material. Ideally a simulation material shouldhave an elongation at yield that is about the same or greater than theelongation at yield of a simulated material.

A third important physical property is the tensile stress. For thepurposes of this invention, a tensile stress for a material that breaksat or before its yield is an important property for simulation purposes.Simulation materials that have a yield stress or break stress (beforeyield) that is lower than the lowest yield stress or break stress(before yield) of a simulated material are usually unsuitable simulationmaterials.

Another important physical property is that of the sense of feel forinherent toughness. The Izod impact strength provides a good measure ofthe toughness of a material. A good simulation material will havetoughness in a range that is close to that of the simulated material.For the discussion herein, the impact strength is measured by thenotched Izod test, according to ASTM Test D-256A.

In general, useful articles are not really used to the point ofbreaking. For example, if a squeeze bottle is made of a material thatbreaks during normal use it will have little value. And in general,useful articles are not often used such that they are stressed pasttheir yield capabilities. For example, if bridges were designed towithstand normal loads, such as a car, which induced stresses in supportmembers exceeding the yield point, the bridge would increase in sag forevery car that passed over it. Exceptions may be found for someapplications such as, for example, living hinges. In these cases, oftenthe first use of the article induces a stress that exceeds the yield ofthe material, but subsequent stresses remain for the most part withinthe elastic range of the material. For the purposes of the instantinvention, material property values relating to the break of a material,for a material having a yield point, are of little interest in terms ofsimulating the look and feel of a simulated material.

The tensile stress usually quoted is the maximum tensile stress, whichis either the stress at yield or the stress at break. If the materialbreaks before it yields, the tensile stress at break of the simulationmaterial should be compared to the tensile yield stress of the simulatedmaterial. If the simulation material exhibits a yield point, the tensileyield stress of the simulation material should be compared to thetensile yield stress of the simulated material. In the case of low tomedium density polyethylene, the tensile stress at yield is 13-28 N/mm2.Simulation materials which have a break tensile stress (before yield) ora tensile yield stress of less than 13 N/mm2, are probably not suitableas a simulation material for low to medium density polyethylene.

The tensile modulus (and/or the flexural modulus) is probably the mostimportant physical property with respect to the feel of a material.People can generally feel the stiffness of a material and can tell ifthe material is not stiff enough or if the material is too stiff. Thisis because the modulus is a material property that is determined in theworking range of a material (i.e. prior to plastic deformation of thematerial) and is a material property that can be felt or measured atrelatively low stress levels. In general, a suitable simulation materialhas a tensile modulus which is within the range of moduli of thesimulated material. Low density to medium density polyethylene has atensile modulus range of from approximately 260 to 520 N/mm2. It hasbeen found that simulation compositions resulting in parts having atensile modulus in the range of about 180 to about 850 N/mm² aresuitable simulation materials. Parts having a modulus below that rangeare generally too soft and pliable to have any utility as a polyethylenesimulation. Conversely, parts having a modulus above that range are toostiff. Preferably, the compositions result in parts having a tensilemodulus in the range of about 220 to about 650 N/mm2.

In the case of the most preferred simulation material for polyethylene,it has been discovered that variations in the exposure during the solidimaging process lead to significant variations in the tensile modulus.For example, by doubling the exposure of this material during the solidimaging process the tensile modulus doubled but the elongation at yieldremained unchanged. This is extremely advantageous for a simulationmaterial since the modulus can be varied over a range that very closelymatches the modulus range of the simulated material. Such a simulationmaterial is therefore adaptable to simulate various molecular weightsand grades of polyethylene, for example.

The elongation properties of a simulation material are also important.If the simulation material has a tensile elongation at break that islower than the minimum tensile elongation at yield of the simulatedmaterial, it is regarded as not suitable. If the material has a yieldpoint, the tensile elongation at yield of the simulation material iscompared with the tensile elongation at yield of the simulated material.If the material does not have a yield point, the tensile elongation atbreak of the simulation material is compared with the tensile elongationat yield of the simulated material. Suitable simulation materials havetensile elongation at yield or tensile elongation at break (beforeyield) values that equal or exceed the tensile elongation at yieldvalues of the simulation material. Low to medium density polyethylenehas a tensile elongation at yield range of 6-8%. Therefore a suitablesimulation material for polyethylene will have a tensile elongation atbreak (before yield) or a tensile elongation at yield of greater than6%.

The impact resistance of a simulation material relative to the impactresistance of a simulated material is also of some importance. Forexample, it is not unusual for someone handling an object to knock theobject against the corner of a table. From such treatment a feel of thematerials toughness and sound qualities (deadening, ringing, etc) can begarnered. For the purposes of this patent, a suitable simulationmaterial will have an Izod impact strength that is nearly as strong asthe Izod impact strength of the simulation material. Low to mediumdensity polyethylene has a notched Izod Impact Strength of 53.4 J/m toNo Break. Therefore a suitable simulation material for low to mediumdensity polyethylene has Notched Izod Impact Strength of at least 50J/m.

The appearance of a simulation material is also an importantconsideration. Low to medium density polyethylene has a cloudyappearance. Therefore a suitable simulation material for low to mediumdensity polyethylene should also have a cloudy appearance and as much aspossible for UV cured materials, minimum color.

The compositions of the invention generally comprise anepoxide-containing material, a free-radical polymerizable aromaticand/or cycloaliphatic acrylic material, a reactive hydroxyl-containingmaterial, a cationic photoinitiator and a free-radical photoinitiator.

The epoxide-containing materials that are used in the compositions,according to this invention, are compounds that possess on average atleast one 1,2-epoxide group in the molecule. By “epoxide” is meant thethree-membered ring

The epoxide-containing materials, also referred to as epoxy materials,are cationically curable, by which is meant that polymerization and/orcrosslinking and other reaction of the epoxy group is initiated bycations. The materials can be monomers, oligomers or polymers and aresometimes referred to as “resins.” Such materials may have an aliphatic,aromatic, cycloaliphatic, araliphatic or heterocyclic structure; theycomprise epoxide groups as side groups, or those groups form part of analicyclic or heterocyclic ring system. Epoxy resins of those types aregenerally known and are commercially available.

The epoxide-containing material (a) may be a solid or a liquid which issoluble or dispersible in the remaining components. It is preferred thatthe epoxide-containing material comprise at least one liquid componentsuch that the combination of materials is a liquid. In the preferredcase, the epoxide-containing material can be a single liquid epoxymaterial, a combination of liquid epoxy materials, or a combination ofliquid epoxy material(s) and solid epoxy material(s) which is soluble inthe liquid.

Examples of suitable epoxy materials include polyglycidyl andpoly(-methylglycidyl) esters of polycarboxylic acids. The polycarboxylicacid can be aliphatic, such as, for example, glutaric acid, adipic acidand the like; cycloaliphatic, such as, for example, tetrahydrophthalicacid; or aromatic, such as, for example, phthalic acid, isophthalicacid, trimellitic acid, or pyromellitic acid. It is likewise possible touse carboxy-terminated adducts, for example, of trimellitic acid andpolyols, such as, for example, glycerol or2,2-bis(4-hydroxycyclohexyl)propane.

Suitable epoxy materials also include polyglycidyl orpoly(-methylglycidyl) ethers obtainable by the reaction of a compoundhaving at least two free alcoholic hydroxy groups and/or phenolichydroxy groups and a suitably substituted epichlorohydrin. The alcoholscan be acyclic alcohols, such as, for example, ethylene glycol,diethylene glycol, and higher poly(oxyethylene) glycols; cycloaliphatic,such as, for example, 1,3- or 1,4-dihydroxycyclohexane,bis(4-hydroxycyclohexyl)methane, 2,2-bis(4-hydroxycyclohexyl)propane, or1,1-bis(hydroxymethyl)cyclohex-3-ene; or contain aromatic nuclei, suchas N,N-bis(2-hydroxyethyl)aniline orp,p′-bis(2-hydroxyethylamino)diphenylmethane.

The epoxy compounds may also be derived from mono nuclear phenols, suchas, for example, from resorcinol or hydroquinone, or they are based onpolynuclear phenols, such as, for example, bis(4-hydroxyphenyl)methane(bisphenol F), 2,2-bis(4-hydroxyphenyl)propane (bisphenol A), or oncondensation products, obtained under acidic conditions, of phenols orcresols with formaldehyde, such as phenol novolacs and cresol novolacs.

Suitable epoxy materials also include poly(N-glycidyl) compounds are,for example, obtainable by dehydrochlorination of the reaction productsof epichlorohydrin with amines that comprise at least two amine hydrogenatoms, such as, for example, n-butylamine, aniline, toluidine,m-xylylene diamine, bis(4-aminophenyl)methane orbis(4-methylaminophenyl)methane. The poly(N-glycidyl) compounds alsoinclude, however, N,N′-diglycidyl derivatives of cycloalkyleneureas,such as ethyleneurea or 1,3-propyleneurea, and N,N′-diglycidylderivatives of hydantoins, such as of 5,5-dimethylhydantoin.

Examples of suitable epoxy materials include poly(S-glycidyl) compoundswhich are di-S-glycidyl derivatives which are derived from dithiols,such as, for example, ethane-1,2-dithiol or bis(4-mercaptomethylphenyl)ether.

Examples of suitable epoxy materials include epoxy compounds in whichthe epoxy groups form part of an alicyclic or heterocyclic ring systemare bis(2,3-epoxycyclopentyl)ether, 2,3-epoxy cyclopentyl glycidylether, 1,2-bis(2,³-epoxycyclopentyloxy)ethane,bis(4-hydroxycyclohexyl)methane diglycidyl ether,2,2-bis(4-hydroxycyclohexyl)propane diglycidyl ether,3,4-epoxycyclo-hexylmethyl-3,4-epoxycyclohexane,3,4-epoxy-6-methyl-cyclohexyl-methyl-3,4-epoxy-6-methylcyclohexanecaboxylate,di-(3,4-epoxycyclohexylmethyl)hexanedioate,di-(3,4-epoxy-6-methyl-cyclohexylmethyl)hexanedioate,ethylenebis(3,4-epoxycyclohexanecarboxylate),ethanedioldi-(3,4-epoxycyclohexylmethyl)ether, vinylcyclohexene dioxide,dicyclopentadiene diepoxide, or2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexane-1,3-dioxane.

It is, however, also possible to use epoxy resins in which the 1,2-epoxygroups are bonded to different hetero atoms or functional groups. Thosecompounds include, for example, the N,N,O-triglycidyl derivative of4-aminophenol, the glycidyl ether glycidyl ester of salicylic acid,N-glycidyl-N′-(2-glycidyloxypropyl)-5,5-dimethylhydantoin, or2-glycidyloxy-1,3-bis(5,5-dimethyl-1-glycidylhydantoin-3-yl)propane.

In addition, liquid pre-reacted adducts of such epoxy resins withhardeners are suitable for epoxy resins.

It is of course also possible to use mixtures of epoxy materials in thecompositions according to the invention.

Preferred epoxy materials are cycloaliphatic diepoxides. Especiallypreferred are bis(4-hydroxycyclohexyl)methane diglycidyl ether,2,2-bis(4-hydroxycyclohexyl)propane diglycidyl ether,3,4-epoxycyclohexylmethyl-3,4 epoxycyclohexanecarboxylate,3,4-epoxy-6-methyl-cyclohexylmethyl-3,4-epoxy-6-methylcyclohexanecarboxylate,di-(3,4-epoxycyclohexylmethyl)hexanedioate,di-(3,4-epoxy-6-methyl-cyclohexylmethyl)hexanedioate,ethylenebis(3,4-epoxycyclohexanecarboxylate),ethanediol-di-(3,4-epoxycyclohexylmethyl) ether, and2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexane-1,3-dioxane.

The epoxy materials can have molecular weights which vary over a widerange. In general, the epoxy equivalent weight, i.e., the number averagemolecular weight divided by the number of reactive epoxy groups, ispreferably in the range of about 60 to about 1000.

The free-radical polymerizable acrylic materials that are used in thecomposition, according to this invention, are compounds that have, onaverage, at least one acrylic group which can be either the free acid oran ester. By “acrylic” is meant the group CH₂=CR¹—CO₂—, where R¹ can behydrogen or methyl. By “(meth)acrylate” is meant an acrylate,methacrylate or combinations thereof. The acrylic materials undergopolymerization and/or crosslinking reactions initiated by free radicals.The acrylic materials can be monomers, oligomers or polymers. It ispreferred that the acrylic material be a monomer or oligomer.

Suitable as the acrylic component are, for example, the diacrylates ofcycloaliphatic or aromatic diols, such as1,4-dihydroxymethylcyclohexane, 2,2-bis(4-hydroxycycyclohexyl)propane,bis(4hydroxycyclohexyl)methane, hydroquinone, 4,4-dihydroxybiphenyl,bisphenol A, bisphenol F, bisphenol S, ethoxylated or propoxylatedbisphenol A, ethoxylated or propoxylated bisphenol F, or ethoxylated orpropoxylated bisphenol S. Such acrylates are known and some of them arecommercially available.

Preferred are compositions comprising as the acrylic component acompound of formula I, II, III or IV

wherein

Y is a direct bond, C1-C6 alkylene, —S—, —O—, —SO—, —SO2—, or —CO—, R10is a C1-C8 alkyl group, a phenyl group that is unsubstituted orsubstituted by one or more C1-C4 alkyl groups, hydroxy groups or halogenatoms, or a radical of the formula CH₂—R11, wherein R11 is a C1-C8 alkylgroup or a phenyl group, and A is a radical of the formula

or comprising as the acrylic component a compound of any one of formulaeVa to Vd,

and the corresponding isomers,

If a substituent is C1-C4 alkyl or C1-C8 alkyl, it may bestraight-chained or branched. A C1-C4 alkyl may be, for example, methyl,ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl or tert-butyl,and a C1-C8 alkyl may additionally be, for example, the various pentyl,hexyl, heptyl, or octyl isomers.

If a substituent is halogen, it is fluorine, chlorine, bromine, oriodine, but especially chlorine or bromine.

If a substituent is C1-C6 alkylene it is, for example, methylene,ethylene, propylene (methylethylene), trimethylene, 1,1-propanediyl,2,2-propanediyl, tetramethylene, ethylmethylene, 1,1-butanediyl,2,2-butanediyl, pentamethylene or hexamethylene. The alkylene radicalsmay also be substituted by halogen atoms. Examples of halogenatedalkylene radicals are —C(CCl₃)₂— and —C(CF₃)₂—.

Especially preferred in the compositions are compounds of the formula V,VI or VII wherein Y is —CH₂— or —(CH₃)₂—. Also especially preferred arecompounds of formula VII wherein R10 is n-butyl, phenyl, n-butoxymethyl,or phenoxymethyl.

Suitable as aromatic tri(meth-)acrylates are, for example, the reactionproducts of triglycidyl ethers of trihydric phenols, and phenol orcresol novolacs having three hydroxy groups with (meth)-acrylic acid.

Compositions wherein the acrylic component is an acrylate of bisphenol Adiepoxide such as Ebecryl® 3700 from UCB Chemical Corporation, Smyrna,Ga. or an acrylate of 1,4-cyclohexanedimethanol are especially preferredfor compositions used in this invention.

In addition to the aromatic or cycloaliphatic acrylic material (b),other acrylic materials can be present. Liquid poly(meth-)acrylateshaving functionality of greater than 2 which may, where appropriate, beused in the compositions according to the invention. These can be, forexample, tri-, tetra-, or penta-functional monomeric or oligomericaliphatic, (meth)acrylates.

Suitable as aliphatic polyfunctional (meth)acrylates are, for example,the triacrylates and trimethacrylates of hexane-2,4,6-triol, glycerol,or 1,1,1-trimethylolpropane, ethoxylated or propoxylated glycerol, or1,1,1-trimethylolpropane and the hydroxy group-containingtri(meth)acrylates which are obtained by the reaction of triepoxycompounds, such as, for example, the triglycidyl ethers of the mentionedtriols, with (meth)acrylic acid. It is also possible to use, forexample, pentaerythritol tetra-acrylate, bistrimethylolpropanetetra-acrylate, pentaerythritol monohydroxytri(meth)acrylate, ordipentaerythritol monohydroxypenta(meth)acrylate.

It is also possible to use hexafunctional urethane (meth)acrylates.Those urethane (meth)acrylates are known to the person skilled in theart and can be prepared in known manner, for example by reacting ahydroxy-terminated polyurethane with acrylic acid or methacrylic acid,or by reacting an isocyanate-terminated prepolymer with hydroxyalkyl(meth)acrylates to follow the urethane (meth)acrylate. Also useful are(meth)acrylates such as tris(2-hydroxyethyl)isocyanurate triacrylate.

The reactive hydroxyl-containing material which is used in the presentinvention may be any organic material having hydroxyl functionality ofat least 1, and preferably at least 2. The material may be a liquid or asolid that is soluble or dispersible in the remaining components. Thematerial should be substantially free of any groups which inhibit thecuring reactions, or which are thermally or photolytically unstable.

Preferably the organic material contains two or more primary orsecondary aliphatic hydroxyl groups, by which is meant that the hydroxylgroup is bonded directly to a non-aromatic carbon atom. The hydroxylgroup may be internal in the molecule or terminal. Monomers, oligomersor polymers can be used. The hydroxyl equivalent weight, i.e., thenumber average molecular weight divided by the number of hydroxylgroups, is preferably in the range of about 31 to 5000.

Representative examples of suitable organic materials having a hydroxylfunctionality of 1 include alkanols, monoalkyl ethers ofpolyoxyalkyleneglycols, monoalkyl ethers of alkylene-glycols, andothers.

Representative examples of useful monomeric polyhydroxy organicmaterials include alkylene and arylalkylene glycols and polyols, such as1,2,4-butanetriol, 1,2,6-hexanetriol, 1,2,3-heptanetriol,2,6-dimethyl-1,2,6-hexanetriol,−(2R,3R)-(−)-2-benzyloxy-1,3,4-butanetriol, 1,2,3-hexanetriol,1,2,3-butanetriol, 3-methyl-1,3,5-pentanetriol, 1,2,3-cyclohexanetriol,1,3,5-cyclohexanetriol, 3,7,11,15-tetramethyl-1,2,3-hexadecanetriol,2-hydroxymethyl-tetrahydro-pyran-3,4,5-triol,2,2,4,4-tetramethyl-1,3-cyclobutanediol, 1,3-cyclopentanediol,trans-1,2-cyclooctanediol, 1,16-hexadecanediol,3,6-dithia-1,8-octanediol, 2-butyne-1,4-diol, 1,3-propanediol,1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol,1,8-octanediol, 1,9-nonanediol, 1-phenyl-1,2-ethanediol,1,2-cyclohexanediol, 1,5-decalindiol, 2,5-dimethyl-3-hexyne-2,5-diol,2,7-dimethyl-3,5-octadiyne-2-7-diol, 2,3-butanediol,1,4-cyclohexanedimethanol.

Representative examples of useful oligomeric and polymerichydroxyl-containing materials include polyoxyethylene andpolyoxypropylene glycols and triols of molecular weights from about 200to about 10,000; polytetramethylene glycols of varying molecular weight;copolymers containing pendant hydroxy groups formed by hydrolysis orpartial hydrolysis of vinyl acetate copolymers, polyvinylacetal resinscontaining pendant hydroxyl groups; hydroxy-terminated polyesters andhydroxy-terminated polylactones; hydroxy-functionalized polyalkadienes,such as polybutadiene; and hydroxy-terminated polyethers.

Preferred hydroxyl-containing monomers are 1,4-cyclohexanedimethanol andaliphatic and cycloaliphatic mono-hydroxy alkanols.

Preferred hydroxyl-containing oligomers and polymers include hydroxyland hydroxyl/epoxy functionalized polybutadiene, polycaprolactone diolsand triols, ethylene/butylene polyols, and combinations thereof.Preferred examples of polyether polyols are polypropylene glycols ofvarious molecular weights and glycerol propoxylate-B-ethoxylate triol.Especially preferred are linear and branched polytetrahydrofuranpolyether polyols available in various molecular weights, such as forexample 250, 650, 1000, 2000, and 2900 MW.

In the compositions according to the invention, any type ofphotoinitiator that, upon exposure to actinic radiation, forms cationsthat initiate the reactions of the epoxy material(s) can be used. Thereare a large number of known and technically proven cationicphotoinitiators for epoxy resins that are suitable. They include, forexample, onium salts with anions of weak nucleophilicity. Examples arehalonium salts, iodosyl salts or sulfonium salts, such as are describedin published European patent application EP 153904, sulfoxonium salts,such as described, for example, in published European patentapplications EP 35969, 44274, 54509, and 164314, or diazonium salts,such as described, for example, in U.S. Pat. Nos. 3,708,296 and5,002,856. Other cationic photoinitiators are metallocene salts, such asdescribed, for example, in published European applications EP 94914 and94915.

A survey of other current onium salt initiators and/or metallocene saltscan be found in “UV-Curing, Science and Technology”, (Editor S. P.Pappas, Technology Marketing Corp., 642 Westover Road, Stanford, Conn.,U.S.A.) or “Chemistry & Technology of UV & EB Formulation for Coatings,Inks & Paints”, Vol. 3 (edited by P. K. T. Oldring).

Preferred cationic photoinitiators are compounds of formula VI, VII, orVIII below,

[R₁—I—R₂]⁺[Q_(m)]⁻  (VI)

wherein:

R₁, R₂, R₃, R₄, R₅, R₆, and R₇ are each independently of the othersC6-C18 aryl that is unsubstituted or substituted by suitable radicals,

L is boron, phosphorus, arsenic, or antimony,

Q is a halogen atom or some of the radicals Q in an anion

LQ_(m) ⁻ may also be hydroxy groups, and

m is an integer that corresponds to the valence of L plus 1.

Examples of C6-C18 aryl are phenyl, naphthyl, anthryl, and phenanthryl.Any substituents present for suitable radicals are alkyl, preferablyC1-C6 alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl,sec-butyl, isobutyl, tert-butyl, or the various pentyl or hexyl isomers,alkoxy, preferably C1-C6 alkoxy such as methoxy, ethoxy, propoxy,butoxy, pentyloxy, or hexyloxy, alkylthio, preferably C1-C6 alkylthio,such as methylthio, ethylthio, propylthio, butylthio, pentylthio, orhexylthio, halogen, such as fluorine, chlorine, bromine, or iodine,amino groups, cyano groups, nitro groups, or arylthio, such asphenylthio.

Examples of preferred halogen atoms Q are chlorine and especiallyfluorine. Preferred anions LQ_(m) ⁻ are BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻,and SbF₅ ⁻ (OH)⁻.

Especially preferred are compositions comprising as the cationicphotoinitiator a compound of formula VIII wherein R5, R6 and R7 arearyl, aryl being especially phenyl or biphenyl, or mixtures of those twocompounds.

Also preferred are compositions comprising as the cationicphotoinitiator component compounds of formula

[R₈(Fe^(II)R₉)_(c)]_(d) ^(+c)[x]_(c) ^(−d),  (IX)

wherein,

c is 1 or 2,

d is 1,2,3,4 or 5,

X is a non-nucleophilic anion, especially PF₆ ⁻, AsF₆ ⁻,

sbF₆ ⁻, CF₃SO₃ ⁻, C₂F₅SO₃ ⁻, n-C₃F₇SO₃ ⁻, n-C₄F₉SO₃ ⁻,

n-C₆F₁₃SO₃ ⁻, or n-C₈F₁₇SO₃ ⁻, R8 is a pi-arene, and R9 is an anion of api-arene, especially a cyclopentadienyl anion.

Examples of pi-arenes as R8 and anions of pi-arenes as R9 are to befound in published European patent application EP 94915.

Examples of preferred pi-arenes as R8 are toluene, xylene, ethylbenzene,cumene, methoxybenzene, methylnaphthalene, pyrene, perylene, stilbene,diphenylene oxide and diphenylene sulfide. Especially preferred arecumene, methylnaphthalene, or stilbene.

Examples of non-nucleophilic anions X⁻ are FSO₃ ⁻, anions of organicsulfonic acids, of carboxylic acids, or anions LQ_(m) ⁻, as alreadydefined above.

Preferred anions are derived from partially fluoro- orperfluoro-aliphatic or partially fluoro- or perfluoro-aromaticcarboxylic acids, or especially from partially fluoro- orper-fluoro-aliphatic or partially fluoro or perfluoro-aromatic organicsulfonic acids, or they are preferably anions LQ_(m) ⁻.

Examples of anions X are BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻, SbF₅ ⁻(OH)⁻,CF₃SO₃ ⁻, C₂F₅SO₃ ⁻, n-C₃F₇SO₃ ⁻, n-C₄F₉SO₃ ⁻, C₆F₁₃SO₃ ⁻, n-C₈F₁₇SO₃ ⁻,C₆F₅SO₃ ⁻, phosphorus tungstate, or silicon tungstate. Preferred are PF₆⁻, AsF₆ ⁻, SbF₆ ⁻, CF₃SO⁻, C₂F₅SO₃ ⁻, n-C₃F₇SO3⁻, n-C₄F₉SO₃ ⁻,n-C₆F₁₃SO₃ ⁻, and n-C₈F₁₇SO₃

The metallocene salts can also be used in combination with oxidizingagents. Such combinations are described in published European patentapplication EP 126712.

In order to increase the light efficiency, or to sensitize the cationicphotoinitiator to specific wavelengths, such as for example specificlaser wavelengths or a specific series of laser wavelengths, it is alsopossible, depending on the type of initiator, to use sensitizers.Examples are polycyclic aromatic hydrocarbons or aromatic ketocompounds. Specific examples of preferred sensitizers are mentioned inpublished European patent application EP 153904. Other preferredsensitizers are benzo[g,h,i]perylene, 1,8-diphenyl-1,3,5,7-octatetraene,and 1,6-diphenyl-1,3,5-hexatriene as described in U.S. Pat. No.5,667,937. It will be recognized that an additional factor in the choiceof sensitizer is the nature and primary wavelength of the source ofactinic radiation.

In the compositions according to the invention, any type ofphotoinitiator that forms free radicals when the appropriate irradiationtakes place can be used. Typical compounds of known photoinitiators arebenzoins, such as benzoin, benzoin ethers, such as benzoin methyl ether,benzoin ethyl ether, and benzoin isopropyl ether, benzoin phenyl ether,and benzoin acetate, acetophenones, such as acetophenone,2,2-dimethoxyacetophenone, 4-(phenylthio)acetophenone, and1,1-dichloroacetophenone, benzil, benzil ketals, such as benzil dimethylketal, and benzil diethyl ketal, anthraquinones, such as2-methylanthraquinone, 2-ethylanthraquinone, 2-tert-butylanthraquinone,1-chloroanthraquinone, and 2-amylanthraquinone, also triphenylphosphine,benzoylphosphine oxides, such as, for example,2,4,6-trimethylbenzoyldiphenylphosphine oxide (Lucirin® TPO),benzophenones, such as benzophenone, and4,4′-bis(N,N′-dimethylamino)benzophenone, thioxanthones and xanthones,acridine derivatives, phenazene derivatives, quinoxaline derivatives or1-phenyl-1,2-propanedione-2-O-benzoyloxime, 1-aminophenyl ketones or1-hydroxyphenyl ketones, such as 1-hydroxycyclohexyl phenyl ketone,phenyl (1-hydroxyisopropyl)ketone and4-isopropylphenyl(1-hydroxyisopropyl)ketone, or triazine compounds, forexample, 4′″methyl thiophenyl-1-di(trichloromethyl)-3,5 S-triazine,S-triazine-2-(stylbene)-4,6-bis-trichloromethyl, and paramethoxy stiryltriazine, all of which are known compounds.

Especially suitable free-radical photoinitiators, which are normallyused in combination with a He/Cd laser, operating at for example 325 nm,an Argon-ion laser, operating at for example 351 nm, or 351 and 364 nm,or 333, 351, and 364 nm, or a frequency tripled YAG solid state laser,having an output of 351 or 355 nm, as the radiation source, areacetophenones, such as 2,2-dialkoxybenzophenones and 1-hydroxyphenylketones, for example 1-hydroxycyclohexyl phenyl ketone,2-hydroxy-1-(4-(2-hydroxyethoxy)phenyl)-2-methyl-1-propanone, or2-hydroxyisopropyl phenyl ketone (also called2-hydroxy-2,2-dimethylacetophenone), but especially 1-hydroxycyclohexylphenyl ketone. Another class of free-radical photoinitiators comprisesthe benzil ketals, such as, for example, benzil dimethyl ketal.Especially an alpha-hydroxyphenyl ketone, benzil dimethyl ketal, or2,4,6-trimethylbenzoyldiphenylphosphine oxide is used as photoinitiator.

Another class of suitable free radical photoinitiators comprises theionic dye-counter ion compounds, which are capable of absorbing actinicrays and producing free radicals, which can initiate the polymerizationof the acrylates. The compositions according to the invention thatcomprise ionic dye-counter ion compounds can thus be cured in a morevariable manner using visible light in an adjustable wavelength range of400 to 700 nanometers. Ionic dye-counter ion compounds and their mode ofaction are known, for example from published European patent applicationEP 223587 and U.S. Pat. Nos. 4,751,102, 4,772,530 and 4,772,541. Theremay be mentioned as examples of suitable ionic dye-counter ion compoundsthe anionic dye-iodonium ion complexes, the anionic dye-pyryllium ioncomplexes and, especially, the cationic dye-borate anion compounds ofthe following formula

wherein D⁺ is a cationic dye and R₁₂, R₁₃, R₁₄, and R₁₅ are eachindependently of the others arkyl, aryl, alkaryl, allyl, aralkyl,alkenyl, alkynyl, an alicyclic or saturated or unsaturated heterocyclicgroup. Preferred definitions for the radicals R₁₂ to R₁₅ can be found,for example, in published European patent application EP 223587.

Especially preferred is the free-radical photoinitiator 1-hydroxyphenylketone, which produces parts having the least amount of yellowing afterfinal cure and provides articles which most closely simulatepolyethylene.

Other additives which are known to be useful in solid imagingcompositions may also be present in the composition of the invention.Stabilizers are often added to the compositions in order to prevent aviscosity build-up during usage in the solid imaging process. Thepreferred stabilizers are described in U.S. Pat. No. 5,665,792. Suchstabilizers are usually hydrocarbon carboxylic acid salts of group IAand IIA metals. Most preferred examples of these salts are sodiumbicarbonate, potassium bicarbonate, and rubidium carbonate. Rubidiumcarbonate is preferred for formulations of this invention withrecommended amounts varying between 0.0015 to 0.005% by weight ofcomposition. Alternative stabilizers are polyvinylpyrrolidones andpolyacrylonitriles. Other possible additives include dyes, pigments,fillers, wetting agents, photosensitizers for the free-radicalphotoinitiator, leveling agents, surfactants and the like.

The compositions of the invention generally comprise from about 20% byweight to less than about 40% by weight of the epoxide-containingmaterial, based on the total weight of the composition.

It is sometimes beneficial to describe the compositions in terms ofequivalents or milliequivalents of epoxy material per 100 grams of totalcomposition. The epoxy equivalent weight can be derived by dividing thenumber of epoxy groups contained within a molecule by the molecularweight of the molecule. The total epoxy equivalent wieght of acomposition is determined by first calculating the epoxy content of eachcomponent, i.e., epoxide-containing material, epoxy-acrylate, etc. Theindividual component epoxy equivalent weihgts are weight averaged forthe entire composition. It is preferred that the compositions comprisefrom about 250 to about 350 milliequivalents of epoxy per 100 grams ofcomposition.

The compositions of the invention preferably comprise from about 5% toabout 45% by weight of free-radical polymerizable acrylic material,based on the total weight of the composition. It is most preferred thatthe acrylic be an aromatic and/or cycloaliphatic diacrylate ordi-methacrylate.

The compositions of the invention preferably comprise from about 5% toabout 50% by weight of reactive hydroxy-containing material, based onthe total weight of the composition, more preferably from about 20% toabout 30% by weight.

It is sometimes beneficial to describe the compositions in terms ofequivalents or milliequivalents of hydroxyl-containing material per 100grams of total composition. The hydroxyl equivalent weight can bederived by dividing the number of hydroxyl groups contained within amolecule by the molecular weight of the molecule. The total number ofequivalent of equivalent of hydroxyl in a composition is determined byfirst calculating the hydroxyl content of each component, i.e.,epoxide-containing material, epoxy-acrylate, polyol, initiator, etc. Theindividual component hydroxyl equivalent weights are weight averaged forthe entire composition. All hydroxyls are assumed to be reactive,regardless of steric hindrance. It is preferred that the compositionscomprise from about 140 to about 180 milliequivalents of reactivehydroxy-containing material per 100 grams of composition. It is mostpreferred that the ratio of epoxy equivalents to hydroxyl equivalents bein the range from about 1.5 to about than 2.5; most preferably 1.9 to2.4.

The compositions of the invention preferably comprise from about 0.2 toabout 10% by weight of cationic photoinitiator, based on the totalweight of the composition.

The compositions of the invention preferably comprise from about 0.01 toabout 10% by weight of free-radical photoinitiator, based on the totalweight of the composition.

The compositions of the invention can be prepared according toconventional procedures. In general, the components are combined bymixing in any suitable mixing apparatus. In some cases, some componentscan be premixed before adding to the total composition. In some cases,the mixing is carried out in the absence of light. In some cases, themixing is carried out with some heating, generally at temperatures thatrange from about 30° C. to about 60° C.

The process for producing three-dimensional articles from thecompositions of the invention, as discussed above, generally involvesexposure of successive thin layers of the liquid composition to actinicradiation. A thin layer of the photosensitive composition of theinvention is coated onto a surface. This is most conveniently done ifthe composition is a liquid. However, a solid composition can be meltedto form a layer. The thin layer is then exposed imagewise to actinicradiation. The radiation must provide sufficient exposure to causesubstantial curing of the photosensitive composition in the exposedareas. By “substantial curing” it is meant that the photosensitivecomposition has reacted to an extent such that the exposed areas arephysically differentiable from the unexposed areas. For liquid, gel orsemi-solid photosensitive compositions, the cured areas will havehardened or solidified to a non-fluid form. For solid photosensitivecompositions, the exposed areas will have a higher melting point thanthe non-exposed areas. Preferably, the exposure is such that portions ofeach successive layer are adhered to a portion of a previously exposedlayer or support region, or to portions of a platform surface. Thisfirst exposure step forms a first imaged cross-section. An additional(second) thin layer of photosensitive composition is then coated ontothe first imaged cross-section and m imagewise exposed to actinicradiation to form an additional (second) imaged cross-section. Thesesteps are repeated with the “nth” thin layer of photosensitivecomposition being coated onto the “(n−1)th” imaged cross-section andexposing to actinic radiation. The repetitions are carried out asufficient number of times to build up the entire three-dimensionalarticle.

The radiation is preferably in the range of 280-650 nm. Any convenientsource of actinic radiation can be used, but lasers are particularlysuitable. Useful lasers include HeCd, argon, nitrogen, metal vapor, andNdYAG lasers. The exposure energy is preferably in the range of about10-75 mJ/cm². Suitable methods and apparatus for carrying out theexposure and production of three-dimensional articles have beendescribed in, for example, U.S. Pat. Nos. 4,987,044, 5,014,207, and5,474,719, which teaches the use of pseudoplastic, plastic flow,thixotropic, gel, semi-solid and solid photopolymer materials in thesolid imaging process.

In general, the three-dimensional article formed by exposure to actinicradiation, as discussed above, is not fully cured, by which is meantthat not all of the reactive material in the composition has reacted.Therefore, there is often an additional step of more fully curing thearticle. This can be accomplished by further irradiating with actinicradiation, heating, or both. Exposure to actinic radiation can beaccomplished with any convenient radiation source, generally a UV light,for a time ranging from about 10 to over 60 minutes. Heating isgenerally carried out at a temperature in the range of about 75-150° C.,for a time ranging from about 10 to over 60 minutes.

EXAMPLES

The components 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate (epoxy); polytetrahydrofuran linear chain (1000 mw)(polyTHF); 1,4-cyclohexanedimethanol (CHDM); 1-hydroxycyclohexyl phenylketone (free-radical initiator, FRI-1);2-hydroxy-1-{4-(2-hydroxyethoxy)phenyl}-2-methyl-1-propanone(free-radical initiator, FRI-2); trimethylolpropane triacrylate (TMPTA);and 2,2-dimethoxy-2-phenylacetophenone (free-radical initiator, FRI-3)are available from Aldrich Chemical Company Inc (Milwaukee, Wis.). Theacrylate ester of bisphenol-A epoxy (Ebecryl) is available from UCBChemicals Corp. (Smyrna, Ga.) as Ebecryle® 3700. The1,4-cyclohexanedimethanol diacrylate esters (CHDMDA) are sold bySartomer Company (Exton, Pa.). The mixed triarylsulfoniumhexafluoroantimonate salts in 50% by weight propylene carbonate(cationic initiator, CatI) is sold by Union Carbide Chemicals andPlastics Company Inc. (Danbury, Conn.). Somos® 2100, 3110, and 7100 arecommercial products sold by E. I. du Pont de Nemours and Company(Wilmington, Del.).

The individual components were weighed out, combined, then heated to 50°C. and mixed for several hours until all the ingredients were completelydissolved.

For all formulations, the exposure-working curve of the formula wasdetermined using methods well known in the art. The working curve is ameasure of the photospeed of the particular material. It represents therelationship between the thickness of a floating layer, scanned on thesurface of the photopolymer in a vat or petri-dish, produced as afunction of the exposure given. Parts were fabricated by forming aseries of 6 mil (0.15 mm) coated layers, and giving enough imagewiseexposure to each layer to create a cure that would correspond to a 10mil (0.254 mm) working curve thickness.

The exposures used to create the tensile and Izod impact test parts aregiven in Table 2. Unless otherwise indicated, all parts were fabricatedusing an Argon Ion laser operating with an output of 351 nm.

After the parts were formed, they were cleaned in a solvent, allowed todry and then fully cured. All parts, except the Somos® 2100 part, weregiven a UV postcure for 60 minutes in a Post Curing Apparatus,manufactured by 3D Systems, Inc. (Valencia, Calif.). The Somos® 2100part was postcured in an oven for 30 minutes at 150 degrees C. then UVpostcured for 30 minutes.

All tensile properties were measured according to ASTM Test D-638M. Forthe Somos® samples, the temperature and humidity were controlled asspecified. The temperature and humidity of the Example parts were-notcontrolled during testing. However, the temperature was approximately20-22° C. and the humidity was approximately 20-30% RH.

The impact stength of all the samples was measured by the knotched Izodtest, according to ASTM Test D-256A.

The physical test values for polyethylene were obtained from varioussources. The values of Tensile Stress at Break, Tensile Yield Stress,Tensile Elongation at Break, Notched Izod Impact, and Tensile Modulusare for polyethylene and ethylene copolymers (low and medium density,linear copolymer) as listed in Modern Plastics Encyclopedia '98,Mid-November 1997 Issue, The McGraw-Hill Companies, Inc., New York, N.Y.The values of Tensile Elongation at Yield for polyethylene were obtainedfrom the Internet address http://www.chemopetrol.cz/docs/en_catal/published by Chemopetrol in Czechoslovakia. The value ranges weredetermined by incorporating the information from Chemopetrol's variousLinten and Mosten grades of polyethylene offerings. The range ofelongation at yield for polyethylene may be greater than that providedin Chemopetrol's product literature.

Examples 1-6

Compositions according to the invention were prepared having thefollowing components, where “Meq epoxy” indicates the number ofmilliequivalents of epoxy per 100 grams of composition, “Meq hydroxyl”indicates the number of milliequivalents of hydroxyl per 100 grams ofcomposition, and oration indicates the ratio of milliequivalents ofepoxy to milliequivalents of hydroxyl:

Parts by weight Component Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Epoxy37.00 37.30 39.50 39.65 36.38 37.38 Ebecryl 25 25 25 25 25 25 PolyTHF 2525 25 25 28.5 27.5 CHDM FRI-1 4 — — — 2.8 2.8 FRI-2 — 3.3 — — — — FRI-3— — 1.2 1.45 — — CHDMA 7 7 7 7 7 7 CatI 2 2.4 2.3 1.9 0.32 0.32 Meqepoxy 274 276 293 294 269 277 Meq hydroxyl 165 175 145 145 166 164 Ratio1.66 1.58 2.01 2.02 1.62 1.69

The compositions of the invention were exposed and tested as describedabove. Examples 1-4 were exposed at 351 nm. Examples 5 and 6 wereexposed at 325 nm. The results are given in Table 1 below.

Comparative Examples C1-C2

Comparative compositions were prepared having the following components,where “Meq epoxy” indicates the number of milliequivalents of epoxy per100 grams of composition, “Meq hydroxyl” indicates the number ofmilliequivalents of hydroxyl per 100 grams of composition, and “Ratio”indicates the ratio of milliequivalents of epoxy to milliequivalents ofhydroxyl:

Parts by weight Component C1 C2 Epoxy 50.20 55.00 Ebecryl 10 — PolyTHF20 20 CHDM 2.5 6.0 FRI-2 3.0 3.5 CatI 2.3 2.5 TMPTA 12 13 Meq epoxy 372407 Meq hydroxyl 140 155 Ratio 2.66 2.64

The compositions were exposed and tested as described above. Samples ofSomos® 2100, 3100 and 3110 were similarly exposed and tested.Comparative Examples 1 and 2 and Somos® 7100 were exposed at 351 nm.Samples of Somos® 3110 were exposed at 325 nm. Samples of Somos® 2100were exposed using a combination of 351 and 364 nm radiation using anArgon ion laser operating with two-line output. The results are given inTable 1 below.

TABLE 1 Property PE Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 TYS 13-28 27 23 24 2817 TMod 260-520 275-620 533 625 820 400 TYE % 6-8 21.4 24.9 26.6 25.4 18Izod 53.4+ 57 70 N/A 66 N/A Energy N/A 31 50 35 36 47 Property Ex. 6 C1C2 2100 3110 7100 TYS 24 33 45 9 26 62 TMod 745 1282 1651 90 1282 2068TYE % 17 4.8 4.7 37 5.7 4.7 Izod N/A N/A 52 90 16 29 Energy 48 43 41 1318 59 TYE = tensile yield stress in N/mm² TMod = tensile modulus inN/mm² TYE % = tensile yield elongation; for all samples the yieldelongation was less than or equal to the break elongation Izod = notchedIzod impact strength in J/m Energy = energy to produce a 0.01 inch(0.254 mm) layer in mJ/cm² N/A = not tested

The formulations in Examples 1 and 5 produced hazy parts that lookedjust like low to medium density polyethylene. Lower exposures, such as,for example, 31 mJ/cm2, during the solid imaging process, produced partsthat were hazier and softer, similar to lower density polyethylenes.Higher exposures, such as, for example, 62 mJ/cm², during the solidimaging process, produced parts that had less haze and were harder,similar to medium density polyethylenes. The tensile stress at yield wasfavorable for Examples 1 and 5 as a simulation material forpolyethylene. The tensile modulus range for Example 1 was a surprisinglygood match for polyethylene. Example 5 will exhibit a similar match forthe tensile modulus range of polyethylene. The tensile break and yieldstrain values of articles made from Examples 1 and 5 were more thanadequate in order to simulate polyethylene articles. The notched Izodimpact strength of Example 1 parts was near the low end, but within therange of polyethylenes. Overall, both were excellent'simulationmaterials for polyethylene.

The formulations from Examples 2-4 and 6 also met most of the criteriato be good simulation materials for polyethylene. The tensile moduliiwere higher than the polyethylene range, but acceptable as a simulationmaterial. Lower exposures during the solid imaging phase of fabricationwould result in lowering the modulus allowing all to be good candidatesas simulation materials for polyethylene.

The formulations in Examples 5 and 6 were variations of Example 1wherein the compositions contained initiator concentrations suitable foruse with a He—Cd laser operating at 325 nm in a solid imaging process.The variations in the Examples involved increases in the3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate content witha concurrent decrease in the polytetrahydrofuran linear chain (1000 MW)content. As can be seen from Table 1, small variations in the3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate content havea remarkable effect on the tensile modulus. This effect is showngraphically in FIG. 2.

Parts made from the formulations of Comparative Examples C1 and C2 werevery stiff. The tensile yield stress and tensile modulus for bothComparative Examples was much higher than that of polyethylene. Inaddition, the tensile yield elongation was unacceptably low. Therefore,Comparative Examples C1 and C2 are not acceptable as simulationmaterials for polyethylene.

Somos® 2100 parts generally appeared milky white. On an appearance basistherefore, it is not desirable as a simulation material forpolyethylene. When comparing the yield and break tensile stressmeasurements for Somos® 2100 against the corresponding values forpolyethylene, it can be seen that Somos® 2100 is too weak. Therefore,Somos® 2100 is regarded as not suitable as a simulation material forpolyethylene. Parts made from Somos® 3110 and 7100 were significantlymore clear than those made of polyethylene and the parts made from thesematerials had tensile modulii that were much too high. In addition, thetensile yield elongation was too low to simulate polyethylene.Therefore, these commerical products also are regarded as not suitableas simulation materials for polyethylene.

What is claimed is:
 1. A photosensitive composition comprising: (a) acycloaliphatic diepoxide; (b) a polyfuntional (meth)acrylate selectedfrom the group consisting of 1,4 cyclohexanedimethanol diacrylate,dipentaerythritol monohydroxypenta(meth)acrylate, and trimethylolpropanetriacrylate; (c) a polytetrahydrofuran polyether polyol having amolecular weight in the range of 250 to 2900; (d) at least one cationicphotoinitiator; (e) at least one free-radical photoinitiator; and (f) anacrylate of bisphenol A diepoxide.
 2. The composition of claim 1,wherein the molecular weight of said polytetrahydrofuran polyetherpolyol is about 1,000.
 3. The composition of claim 1, wherein saidcomposition, after full cure by radiation and optionally heat, has atensile modulus in the range of 180 to 850 N/mm².
 4. The composition ofclaim 1, wherein said composition, after full cure by radiation andoptionally heat, has a tensile break elongation before yield or atensile yield elongation greater than 6%.
 5. The composition of claim 1,wherein said composition, after full cure by radiation and optionallyheat, has a notched Izod impact strength greater than 50 J/m.
 6. Thecomposition of claim 1, wherein said composition, after full cure byradiation and optionally heat, has a tensile break before yield stressor a tensile yield stress greater than 13 N/mm².